Semipolar double bonds

Lowry, the holder of the new chair in physical chemistry at Cambridge University, spoke in Paris in March 1924 on aspects of the theory of valence, including the electronic theory of valence, and in December 1925 on optical methods of verifying structural chemistry and his own hypothesis of semipolar double bonds in organic compounds. The occasions were meetings of the Societe de Chimique de France and the Societe de Chimie Physique. 62... 

The second formula means merely that the HC1 molecule is a resonance hybrid between the ionic molecule H+Cl" and the molecule with the purely covalent bond, the direction of the arrow giving the direction in which the electrons have, on the average, been displaced (66). As, however, such an arrow is used by others (57), for indicating a coordinate link (semipolar double bond) caused by a lone electron pair of the donor atom, which likewise produces a dipole with its positive end on the donor side and its negative one on the acceptor side, the author suggests that the symbol — be used for the normal covalent bond, which, by resonance with an ionic structure, possesses a dipole. The point of this half arrow also indicates the direction of the negative end of the dipole. The full arrow — will then be reserved for the coordinate link. Both links play their roles in chemisorption, and it may be useful for the purposes of this article to introduce relatively simple symbols. According to this principle HC1 should be formulated as H—1-Cl. 

In all these cases the oxygen atom has a negative charge, while the nitrogen or sulphur atom carries a corresponding positive charge. In the old formulae double bonds were used, later also semipolar double bonds there is, however, here no difference in principle with the normal single electron pair bond. 

Sugden has called this type of union a semipolar double bond. 

From this, it is sometimes called a semipolar double bond - a combination of a single ionic bond and a single covalent bond. 

We will here refer to the kind of bond formed in (1) as a Lewis bond. They are also commonly called coordinate-covalent bonds or semipolar double bonds. The hydrogen bond (2) is also a specihc type of Lewis bond, and is the type of Lewis bond that is most commonly encountered in organic molecules. 

Although the S—O bond lengths in sulphoxides and sulphones seem to indicate that these are covalent double bonds, the dipole moments of these compounds obtained by calculation or by experimental measurements support the semipolar single-bond character in these compounds, and they should be represented as S - O, with about 66% ionic character14,15. 

On the basis of these values one can conclude that, with increasing bond orders, the force constants rise, suggesting that the S—O bond of sulphoxides should have more semipolar character than that of sulphones. Furthermore, molecular diffraction measurements20 and Parachors21 for sulphoxides also suggest that the S—O bond in sulphoxides should have a semipolar single-bond representation while the S—O bond in sulphones is described by double bonds or better as the resonance hybride shown in Scheme 1. 

Since all these states have the same number of double bonds and semipolar bonds, the energy of the four states will be almost equal, and the different states will occur with almost the same frequency. 

Bromine forms the same type of compounds, viz. the very unstable hypobromites AOBr and the bromates ABr03. In all these reactions the double bonds in the oxides become semipolar bonds in the complex ions. 

As this formula indicates in the nitro group one atom of oxygen is linked to the nitrogen atom by a double bond, the other-by a semipolar linkage (see Chapter V on the structure of the nitro group, p. 168). Bond distances and bond angles have been calculated by Maxwell and Mosley [5] (Fig. la), using their own experiments... 

The differences have the sign and magnitude which is associated with a mixed bond, and this confirms the structure assigned on p. 52. The ethyl ester of phenylmethylphosphinic acid had a paraehor of 420-5 that calculated on the assumption of an ordinary double bond was 442-1, while on the assumption of a mixed or semipolar bond it was 417-3. The structure was therefore given as... 

Orient formulae so that the principal heteroatom is at the bottom and the numbering proceeds counterclockwise around the ring. Always write in double bonds, do not use circles in rings. For mesoionic compounds, give (one of) the most important canonical forms. Do not use semipolar bonds, i.e., use —0 , and not N - O for fV-oxides. 

The structure of [(CH3)2As]2S2 is shown in Fig. 8 402). This compound, cacodyl disulfide, seems to have been assumed to have a disulfide link as in (CH3)2As—S—S—As(CH3)2. However, chemical evidence such as its formation from [(CH3)2As]2S and S, was against such a formulation. The correct structure is in accord with the chemical properties of the compound 178,402). The differences in the three As—S distances are significant, the short 2.01-A bond indicating double-bond (or semipolar) character, and the 2.21-A bond possibly indicating a smaller covalent radius for As(V) than for As(III). The As—S distances in AS4S6 and AS4S4 are 2.25 A and 2.33 A... 

The SS bond in thiosulfoxides may be considered as either double or semipolar depending on the electronegativity of the substituents [25] see Scheme 1. 

In this process an electron pair, shared by a N and an O atom, is moved completely to the O atom. This is equivalent to the transfer of one electron from the N to the O atom, and by this process the N atom acquires a positive charge of one unit, whereas the oxygen atom obtains a negative charge the double covalent bond is transformed into one single covalent bond plus an ionic one. This type of bond is called a semifiolar bond. By assuming a sufficient number of semipolar bonds, all the other oxides with an even number of... 

In Section 54 ample evidence was given for the existence of stable multiple bonds in the oxides of group II. In this group, sulphur forms a transition to the following group. In the lower oxide SOs two double or semipolar bonds must be assumed... 

The structure SeOs in the gaseous or solid state is unknown, but in the lower oxide there is polymerization. In Se02 there are two double or semipolar bonds, but in the solid state the compound forms chains... 

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